EP 1 555 059 A1 discloses a liquid filter for internal combustion engines that is embodied as an additive filter and is provided with an additive that, when the filter element is flowed through, releases active ingredients into the fluid. The active ingredients are designed to functionally improve the properties of the fluid. One embodiment of this reference shows a cylindrical filter element that is comprised of a zigzag-folded filter medium in a cylindrical arrangement wherein an additive has been introduced into some of the folds of the filter element that are open in a radial outward direction. The filter element is flowed through by the liquid to be purified in radial direction from the exterior to the interior so that the exterior side is the raw side (unfiltered side) and the liquid is brought into direct contact with the additive that is open toward the raw side.